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United States Patent |
5,257,042
|
Buhler
|
October 26, 1993
|
Thermal ink jet transducer protection
Abstract
The present invention provides an ink jet printhead that is provided a bias
voltage and that includes at least one ink channel, a heating element, and
an interconnect. The ink channel has an open end that serves as a nozzle,
and the heating element is positioned in the channel for ejecting ink
droplets from the nozzle by selective application of current pulses along
the interconnect to the heating element. The printhead further includes a
conductive protective region that is positioned adjacent the heating
element and that has a portion thereof exposed to the ink channel for
protecting the heating element from ink. Positioned between the conductive
protective region and the heating element is a dielectric region for
insulating the heating element from the conductive protective region. The
printhead also includes a bus for connecting the bias voltage to the
conductive protective region.
Inventors:
|
Buhler; Steven A. (Redondo Beach, CA)
|
Assignee:
|
Xerox Corporation (Stamford, CT)
|
Appl. No.:
|
727493 |
Filed:
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July 9, 1991 |
Current U.S. Class: |
347/64; 347/58 |
Intern'l Class: |
B41J 002/05 |
Field of Search: |
346/140 R
|
References Cited
U.S. Patent Documents
Re32572 | Jan., 1988 | Hawkins et al. | 156/626.
|
4370668 | Jan., 1983 | Hara et al. | 346/140.
|
4532530 | Jul., 1985 | Hawkins | 346/140.
|
4719477 | Jan., 1988 | Hess | 346/140.
|
4774530 | Sep., 1988 | Hawkins | 346/140.
|
4931813 | May., 1990 | Pan et al. | 346/140.
|
4935752 | Jun., 1990 | Hawkins | 346/140.
|
4947192 | Aug., 1990 | Hawkins et al. | 346/140.
|
4947193 | Aug., 1990 | Deshpande | 346/140.
|
4951063 | Aug., 1990 | Hawkins et al. | 346/1.
|
4956653 | Sep., 1990 | Braun | 346/140.
|
Primary Examiner: Fuller; Benjamin R.
Assistant Examiner: Bobb; Alrick
Attorney, Agent or Firm: O'Neill; Daniel J.
Claims
I claim:
1. In an ink jet printhead having at least one ink channel, a heating
element, and an interconnect, the ink channel having an open end that
serves as a nozzle, the heating element being positioned in the ink
channel for ejecting ink droplets from the nozzle by selective application
of current pulses along the interconnect to the heating element, said
printhead further comprising:
ink contained within the ink channels;
a conductive protective region positioned adjacent the heating element and
having a portion thereof exposed to the ink channel for protecting the
heating element from said ink;
a dielectric region positioned between the heating element and said
conductive protective region for electrically insulating the heating
element from said conductive protective region;
a bias voltage having a magnitude equal to or less than the difference in
work functions of the conductive protective region and the ink; and
means for connecting said bias voltage to said conductive protective region
so that said conductive protective region is provided with anodic
protection.
2. The thermal ink jet printhead of claim 1, wherein said conductive
protective region includes tantalum, and said bias voltage has a magnitude
of less than 1 volt.
3. A thermal ink jet printhead supplied with a bias voltage sufficient to
provide anodic protection and having an ink channel structure with a
plurality of nozzles at one end, an ink manifold at another end, and a
plurality of ink channels with an ink channel connecting each nozzle to
the ink manifold, the ink channel structure fixedly adjoined to a circuit
board which contains driver logic and heating elements formed on a surface
of a common substrate, the heating elements being positioned in the
channels for ejecting ink droplets from the nozzles, said printhead
further comprising:
a conductive protective region positioned adjacent each of the heating
elements and having a portion thereof exposed to the ink channel for
protecting the heating element from ink;
a dielectric region positioned between each of the heating elements and
their respective conductive protective regions for electrically insulating
each of the heating elements from their respective conductive protective
regions; and
means for connecting the bias voltage to said conductive protective regions
so that said conductive protective regions are provided with anodic
protection, said bias voltage connecting means further including a
conductive interconnect, made of aluminum, for connecting the bias voltage
to said conductive protective regions, said conductive interconnect being
the bottom metal level of a double metal process.
4. A thermal ink jet printer having a printhead having a plurality of
nozzles at one end, an ink manifold at another end, and a plurality of ink
channels with an ink channel connecting each nozzle to the ink manifold,
and heating elements being positioned in the ink channels for ejecting ink
droplets from the nozzles upon selected application of current pulses to
the heating elements, the printer further comprising:
conductive, grounded ink contained in the ink manifold and ink channels;
means for supplying a bias voltage sufficient to provide anodic protection;
a conductive protective region positioned adjacent each heating element and
having a portion thereof exposed to the ink channel for protecting the
heating element from ink;
a dielectric region positioned between each of the heating elements and
their respective conductive protective regions for electrically insulating
each of the heating elements from their respective conductive protective
regions; and
means for connecting said bias voltage supply means to said conductive
protective regions, said bias voltage supply connecting means including a
conductive path through said ink to ground.
5. A thermal ink jet printer having a printhead having a plurality of
nozzles at one end, an ink manifold at another end, and a plurality of ink
channels with an ink channel connecting each nozzle to the ink manifold,
and heating elements being positioned in the ink channels for ejecting ink
droplets from the nozzles upon selected application of current pulses to
the heating elements, the printer further comprising:
means for supplying a bias voltage sufficient to provide anodic protection;
a conductive protective region positioned adjacent each heating element and
having a portion thereof exposed to the ink channel for protecting the
heating element from ink;
a dielectric region positioned between each of the heating elements and
their respective conductive protective regions for electrically insulating
each of the heating elements from their respective conductive protective
regions; and
means for connecting said bias voltage supply means to said conductive
protective regions, wherein the heating elements, said conductive
protective regions, and said bias voltage supply connecting means as a
group are constructed such that the group RC time constant is less than
the rise time of a current pulse sent to the heating elements.
6. A thermal ink jet printer having a printhead having a plurality of
nozzles at one end, an ink manifold at another end, and a plurality of ink
channels with an ink channel connecting each nozzle to the ink manifold,
and heating elements being positioned in the ink channels for ejecting ink
droplets from the nozzles upon selected application of current pulses to
the heating elements, the printer further comprising:
conductive ink positioned in the ink channels;
means for supplying a bias voltage sufficient to provide anodic protection;
a conductive protective region positioned adjacent each heating element and
having a portion thereof exposed to the ink channel for protecting the
heating element from ink;
a dielectric region positioned between each of the heating elements and
their respective conductive protective regions for electrically insulating
each of the heating elements from their respective conductive protective
regions; and
means for connecting said bias voltage supply means to said conductive
protective regions, wherein said bias voltage supply means provides said
conductive protective region with a positive bias voltage of between
approximately 0 volts and 1 volt with respect to said conductive ink.
7. The thermal ink jet printer of claim 6, wherein said bias voltage supply
means supplies said conductive protective region with a bias voltage of
approximately 0.5 volts with respect to said conductive ink.
Description
This invention relates to thermal ink jet printheads, and more particularly
to thermal ink jet printheads constructed to resist corrosion of heater
elements.
BACKGROUND AND INFORMATION DISCLOSURE STATEMENT
Thermal ink jet printers are well known in the prior art as exemplified by
U.S. Pat. No. Re. 32,572 issued to Hawkins et al. In the system disclosed
in this patent, a thermal printhead comprises one or more ink-filled
channels communicating with a relatively small ink supply chamber at one
end and having an opening at the opposite end, referred to as a nozzle. A
plurality of heating resistors are located in the channels at a
predetermined distance from the nozzle. The heating resistors are
individually addressed with a current pulse to momentarily vaporize the
ink and form a bubble which expels an ink droplet. Typically, the ink is
water-based, and the bubble that forms consists of water vapor. As the
bubble grows, the ink bulges from the nozzle and is contained by the
surface tension of the ink as a meniscus. As the bubble begins to
collapse, the ink still in the channel between the nozzle and bubble
starts to move towards the collapsing bubble, causing a volumetric
contraction of the ink at the nozzle and resulting in the separating of
the bulging ink as a droplet. The acceleration of the ink out of the
nozzle while the bubble is growing provides the momentum and velocity of
the droplet in a substantially straight line direction towards a recording
medium, such as paper.
In the channels, the heating resistors are subject to wear from corrosive
ink as well as from mechanical shock produced by collapsing bubbles and
thermal fatigue. In particular, the temperature of the ink adjacent an
active heating resistor reaches at least 300 degrees centigrade, which is
the temperature at which bubble nucleation occurs. Since the expected
lifetime for commercial heating resistors is at least 200 million firings,
measures are taken to protect the heating resistors. One measure is to
construct the heating resistors to withstand the wear. For example, U.S.
Pat. No. 4,931,813 to Pan et al. discloses forming the heating resistor
from a relatively thick layer of unpassivated resistive material, such as
TaAl. While this approach is generally adequate, it has the disadvantage
that direct exposure of the heating resistors to the ink and cavitation
forces can cause wear of and changes to the heating resistors. These
effects can result in nonuniform print quality.
Another measure is to cover the heating resistors with protective layers,
thus sparing the resistors from direct contact with the ink. For example,
U.S. Pat. No. Re. 32,572 issued to Hawkins et al, U.S. Pat. No. 4,774,530
to Hawkins and U.S. Pat. No. 4,935,752 to Hawkins disclose covering
heating resistors and associated electrodes with a passivation layer of
silicon dioxide, silicon nitride, or both. In addition, a tantalum layer
may be deposited on the passivation layer above the heating resistors for
additional protection against cavitation forces. Similarly, U.S. Pat. No.
4,951,063 to Hawkins et al. discloses covering heating resistors with a
high temperature deposited plasma or pyrolytic silicon nitride layer
followed by a tantalum layer. Tantalum layers are strong and resist
corrosion.
While the tantalum layer generally provides adequate protection, it is
subject to erosion. One mechanism for erosion is hydrogen embrittlement, a
process whereby a metal, such as tantalum, absorbs hydrogen and becomes
brittle. Brittle tantalum can be easily fractured, particularly since the
tantalum layer is subject to cavitation forces when a bubble collapses.
Hydrogen can be absorbed into many materials if a voltage bias is present.
Moreover, even without a bias voltage, tantalum can absorb hydrogen if the
temperature of the tantalum is sufficiently high. For example, absorption
occurs without bias at the operating temperature of a typical thermal ink
jet. In a typical thermal ink jet, the temperature on the tantalum layer
surface reaches at least 300 degrees centigrade, the temperature at which
bubble nucleation occurs. After nucleation, the temperature exceeds the
nucleation temperature because the heating resistor is still producing
heat and the newly formed bubble insulates the heating resistor from the
heat-conducting ink. The temperature can reach 450 degrees centigrade.
The source of the hydrogen is the hydronium ion (the hydrated proton,
H.sub.3 O.sup.+). The hydronium ion is always present in the water in the
water-based ink. Aside from hydronium ions normally present in water, the
ink typically contains a greater concentration of hydronium ions because
it is salted and acidic. The ink is salted to make it conductive to aid in
sensing the amount, or absence, of ink in a printhead. Moreover, the ink
is made acidic to avoid the etching of tantalum and of silicon that
results from alkaline water.
Another mechanism for erosion of the tantalum layer is electrochemical
reaction between the tantalum and the ink. The reaction is increased by
voltage transients or spikes that pass through the tantalum layer during
the rise and fall of a current pulse through the heating resistor
associated with that particular tantalum layer. The voltage spikes are
caused by capacitive coupling between the tantalum layer and its heating
resistor. Capacitive coupling occurs because the tantalum region is
separated from the heating resistor by an insulating dielectric layer,
forming a capacitor between the tantalum layer and its heating resistor.
Significant capacitive coupling occurs unless the RC time constant of the
tantalum layer and surrounding environment is much less than the rise and
fall times of the current pulses. Typically, the current pulses have a
period of 5 microseconds, and correspondingly short rise and fall times
(e.g., 10 to 50 nanoseconds). The rise and fall times are particularly
quick for printheads having the current pulse driver transistors located
on the same integrated circuit substrate as the heating resistors.
(Placing drive transistors and resistors on the same substrate is popular
because it allows multiplex addressing of the drive transistors, which
reduces the number of leads connected to the substrate. Placing drive
transistors on the same substrate, however, reduces the capacitive load to
the driver transistors, which also decreases the rise and fall times.) For
calculating the RC time constant, typically there is a capacitance of
about 3 picofarads between a tantalum layer and its associated resistor.
The resistive component of the RC time constant is mainly the resistance
from the tantalum layer to ground through the conductive ink contacting
the tantalum layer. The ink resistance depends largely on the salt content
of the ink. Ink resistances range from 1000 ohms to 50,000 ohms, with
10,000 ohms being a typical value. For the typical ink resistance of
10,000 ohms, the RC time constant is about 30 nanoseconds. For this case
the magnitude of the voltage spikes approaches its theoretical maximum of
half the voltage across the heating resistor.
SUMMARY OF THE INVENTION
According the present invention, an ink jet printhead is supplied a bias
voltage and has at least one ink channel, a heating element, and an
interconnect. The ink channel has an open end that serves as a nozzle, and
the heating element is positioned in the channel for ejecting ink droplets
from the nozzle by selective application of current pulses along the
interconnect to the heating element. The printhead further includes a
conductive protective region that is positioned adjacent the heating
element and that has a portion thereof exposed to the ink channel for
protecting the heating element from ink. Positioned between the conductive
protective region and the heating element is a dielectric region for
insulating the heating element from the conductive protective region. The
printhead also includes means for connecting the bias voltage to the
conductive protective region.
In other aspects of the present invention, the protective region includes a
layer of tantalum, and the means for connecting the bias voltage to the
conductive protective region includes an aluminum interconnect for
providing a low resistance connection between the bias voltage and the
conductive protective region.
Other features of the present invention will become apparent as the
following description proceeds and upon reference to the drawings, in
which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged schematic isometric view of a printhead embodying the
invention;
FIG. 2 is an enlarged cross sectional view of the printhead of FIG. 1;
FIG. 3 is an enlarged cross sectional view of the printhead of FIG. 1;
FIG. 4 is a partial schematic top view of the printhead of FIG. 1, showing
the power buses, heating resistors, tantalum protective regions, drive
transistors and control logic; and
FIG. 5 is a partial schematic top view of the printhead of FIG. 1 that
shows the capacitive coupling between heating resistors and their
associated tantalum protective regions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
While the present invention will hereinafter be described in connection
with a preferred embodiment and method of manufacture, it will be
understood that it is not intended to limit the invention to that
embodiment. On the contrary, it is intended to cover all alternatives,
modifications, and equivalents as may be included within the spirit and
scope of the invention as defined by the appended claims.
Referring now to FIGS. 1 and 2, there is shown a preferred embodiment of a
side shooter thermal ink jet (TIJ) printhead 10 embodying the present
invention. Printhead 10 comprises an electrically insulated substrate
heater board 12 permanently attached to a structure board 14. Structure
board 14 includes parallel triangular cross-sectional grooves 16 which
extend from an ink reservior 18 in one direction and penetrate through
front edge of printhead 10. Heater board 12 is aligned and bonded to the
surface of structure board 14 with grooves 16 so that ink channels 20 are
formed by grooves 16 and the surface 22 of the heater board 12, and so
that a respective one of the plurality of ink channels 20 has positioned
in it a respective one of the plurality of heating resistors 24. Ink
reservior 18 can be filled with ink through fill hole 26. The presence of
ink (not shown) in reservoir 18 is detected by a sensor (not shown) that
includes grounded sensor contact 27, positioned on a portion of heater
board 12 that forms the base of reservoir 18.
Referring now to FIGS. 1 and 4, ink drops 28 are ejected from channels 20
along paths 30 in response to current pulses sent to heating resistors 24
by drive transistors 32. Drive transistors 32 are controlled by logic
control section 34. Heating resistors 24, drive transistors 32 and logic
control section 34 are all formed on surface 22 of heater board 12. A
preferred technique for forming drive transistors 32 is by monolithic
integration of MOS transistor switches onto the same silicon substrate
containing heating resistors 24. This technique is described in U.S. Pat.
No. 4,947,192 issued to Hawkins et al., which is incorporated by
reference. In FIG. 1, while only 24 ink channels 20 are shown for
illustrative purposes, it is understood that many more channels 20 may be
formed within a single printhead 10. For page width applications, for
example, printhead 10 may include 200 channels 20.
Referring now to FIGS. 3 and 4, heating resistors 24 are positioned in
close proximity to (about 120 micrometers away from) the front face 36 of
printhead 10. An aluminum power bus 38 extends in the space between front
face 36 and heating resistors 24, and connects to heating resistors 24 by
means of interconnects 41 that are positioned between power bus 38 and
heating resistors 24. Power bus 38 terminates at either end in terminals
40. Via terminals 40, power bus 38 connects to an external power supply
(not shown). At terminals 40, the external power supply typically provides
40 Volts. Connecting opposite ends of bus 38 to the power supply reduces
the voltage drop across along the length of bus caused by parasitic
resistance. Heating resistors 24 are connected to the drains (not shown)
of their respective drive transistors 32 by aluminum interconnects 42.
Interconnects 42 contact their respective heating resistors 24 at the side
25 of heating resistors 24 opposite power bus 38. The sources (not shown)
of drive transistors 32 connect to a common bus 46, and the gates connect
to logic control section 34. Common bus 46 terminates at either end in
terminals 40, via which common bus 46 connects to an external ground (not
shown).
Each heating resistor 24 is covered by a tantalum region 56, and between
each tantalum region 56 and its heating resistor 24 is a dielectric region
54. A passivation layer 61 covers most of the surface 22 of heater board
12. Left uncovered by passivation layer 61 are terminals 40, and a portion
of each tantalum region 56 to allow ink 57 to contact the tantalum regions
56. The tantalum regions 56 and dielectric regions 54 protect their
associated heating resistors 24 from cavitation damage and from the
corrosive effects of ink 57. Moreover, dielectric regions 54 prevent their
associated tantalum regions 56, which are conductive, from shorting their
associated heating resistors 24. Dielectric regions 54 are about 0.5
micrometers thick, and are constructed of silicon dioxide, silicon
nitride, or layers of both materials.
Ink 57 is particularly corrosive because it is salted to make it
conductive. Ink 57 needs to be conductive for proper operation of the
standard types of ink sensors (not shown). The ink sensor senses the
presence or absence of ink in reservoir 18. The ink sensor includes a
sensor contact 27 (shown in FIGS. 1 and 2), positioned on heater board 12
within reservoir 18. Sensor contact 27 is connected to an external ground
(not shown).
Referring now to FIGS. 3 and 5, in accordance with the invention, tantalum
regions 56 are interconnected by means of an aluminum bus 58, and are
connected to the grounded sensor contact 27 by means of conductive ink 57.
Bus 58 extends in the space between tantalum regions 56 and front face 36
of printhead 10. Bus 58 terminates at either end in terminals 40. At
terminals 40, bus 58 connects to an external bias supply 59 that provides
bus 58, and hence tantalum regions 56, with a positive bias with respect
to ink 57. Connecting opposite ends of bus 58 to bias supply 59 reduces
the voltage drop across along the length of bus 58 caused by parasitic
resistance. Of course, external bias supply 59 could be replaced with a
power supply provided internal to printhead 10, such as a battery or a
regulated power supply.
Referring now to FIGS. 1, 3 and 5, both power bus 38 and bus 58 are
constructed in the relatively narrow space between heating resistors 24
and front face 36 of printhead 10. In the preferred embodiment, heater
board 12 is constructed using a two metal process, with bus 58 constructed
in the first metal layer and power bus 38 constructed in the second metal
layer. While a two metal process is more complicated than a single metal
process, it allows power bus 38 and bus 58 to be connected to heating
resistors 24 and tantalum regions 56, respectively, without the need for
higher resistance interconnects, such as doped polysilicon, to bridge over
or under one or the other. Power bus 38 is constructed in the second metal
layer because power bus 38 needs to handle more power than bus 58, and in
a two metal process the second metal layer is thicker than the first metal
layer, and hence more suitable to the power requirements of power bus 38.
In the preferred embodiment, a return path for the positive bias provided
to tantalum regions 56 by bus 58 is provided by conductive ink 57 and the
contact of ink 57 with grounded sensor contact 27. Alternatively, a return
path could be provided by connecting tantalum regions 56 to common bus 46.
The connection between tantalum regions 56 and common bus 46 could be made
using conductive polysilicon interconnections.
Supplying tantalum regions 56 with the appropriate positive bias reduces
hydrogen embrittlement of the tantalum in tantalum regions 56. The
appropriate positive bias provides anodic protection by canceling, or at
least reducing, the difference in work functions between the tanalum in
tantalum regions 56 and the hydrogen ions present in ink 57.
For any given printhead 10, the proper bias should be determined by
experiment. An upper limit on the magnitude of the positive bias is set by
the bias at which electrolysis of the water occurs, which is one volt: For
a positive bias of approximately 1 volt or greater, electrolysis of the
water in the ink takes place, causing bubbles to form in the ink that
degrade performance by absorbing energy that otherwise would be used to
expel droplets 28. Thus, the proper bias determined by experiment is
likely to be between 0 and 1 volt. Based on the difference in work
functions between tantalum and the hydrogen ions, the appropriate positive
bias should be about 0.5 volts.
Interconnecting tantalum regions 56 with a low resistance bus 58 reduces
corrosion of the tantalum regions 56 caused by electrochemical reaction.
Interconnecting the tantalum regions 56 with a low resistance bus 58
reduces capacitive coupling between an active heating resistor 24 and its
tantalum region 56, thereby reducing the magnitude of voltage spikes that
pass through the tantalum region 56 during the rise and fall of the
heating pulse.
The reduction in capacitive coupling can be shown with reference to FIGS. 3
and 5. Like FIG. 4, FIG. 5 is a partial schematic top view of printhead
10. In addition, the FIG. 5 schematic diagram models the capacitive
coupling between a heating resistor 24 and its tantalum region 56. In the
model, each tantalum region 56 is represented by a resistor 65. The
capacitance between each tantalum region 56 and its respective heating
resistor 24 is represented by a capacitor 63. Opposite ends of each
capacitor 63 are connected to the midpoints of its associated heating
resistor 24 and resistor 65, an arrangement that reflects the parasitic
nature of the capacitive coupling between a heating resistor 24 and its
respective tantalum region 56. One end of each resistor 65 is connected to
bus 58. The other end of each resistor 65 is connected to ground (i.e.,
grounded sensor contact 27) through a resistor 67. Resistors 67 provide a
simplified representation of the resistance that ink 57 provides between
each tantalum region 56 and grounded sensor contact 27.
Using the model of FIG. 5, the time constant for a single active heating
resistor 24 can be calculated. The time constant is calculated as the
product of resistances and capacitances in the path connecting active
heating resistor 24 to ground through its respective tantalum region 56.
In this path the only capacitance is capacitor 63, but a few resistances
need to be taken into account. Calculating the resistance of the path is
simplified by recognizing that bus 58 is an AC ground. For simplicity, bus
58 is assumed to be at a DC ground level as wel (a bias of 0 volts). With
bus 58 at ground, the RC time constant path contains a part of the active
heating resistor 24 in series with the parallel combination of ink
resistor 67 and a part of tantalum region resistor 65. Typical resistances
of ink resistor 67 and tantalum region resistor 65 are 10,000 ohms and 15
ohms, respectively. (The value for resistor 65 is derived from the area of
each tantalum region 56, 5 squares, and the sheet resistance of the
tantalum, 3 ohms per square.) Given these relative resistances, the
parallel combination can be approximated as the resistance of part of
resistor 65, or simply the resistance of resistor 65. The resistance
component of the RC time constant is then the sum of the resistance of
resistor 65 and a portion of the resistance of heating resistor 24, or
approximately the sum of the resistances of resistors 24 and 65, or about
200 ohms. The measured capacitance of capacitor 65 is about 3 picofarads.
The resulting time constant is 0.6 nanoseconds, much less than the
measured minimum rise time of 10 nanoseconds. In contrast, for a similar
prior art system lacking bus 58, the time constant would be approximately
the product of the capacitance of capacitor 63 with the sum of the
resistances of resistor 67 and active heating resistor 24, or about 30
nanoseconds.
In calculating time constants, it is important to realize that often up to
four adjacent heating resistors 24 are switched on as a group. These four
active heating resistors 24 possess a group RC time constant that is
approximately four times greater than the time constant of a single active
heating resistor 24. The factor of four reflects the parallel combination
of four capacitors 63; four resistors 67 are not combined in parallel,
despite the model of FIG. 5, since such a combination does not accurately
describe the resistance of ink 57 for the case of four active, adjacent
heating resistors 24.
The model of FIG. 5 presents a simplified view of printhead 10 that is
adequate for demonstrating the effects of capacitive coupling, and showing
how the effects are reduced by bus 58. Of course, the model has certain
limitations (e.g., modeling ink 57 as a series of resistors 67 works well
for analyzing a single active heating resistor 24, but not for analyzing
multiple active heating resistors 24). Moreover, the model assumes that
bus 58 has negligible resistance, and that the only capacitive component
that need be considered is the capacitance between a heating resistor 24
and its associated tantalum region 56. From measurements and calculations,
the latter assumption is correct. Whether the resistance of bus 58 is
negligible, however, depends on the material from which bus 58 is
constructed. Preferably, bus 58 is made of aluminum, a material that
typically has a sheet resistance of 0.03 ohms per square. The resistance
of an aluminum bus 58 is negligible compared to the resistances of ink 57
or tantalum regions 56. However, were bus 58 to be made of other materials
commonly used to make connections for integrated circuits, the resistance
of bus 58 may be a factor. For example, the resistance of bus 58 would be
a factor were it made from either tantalum or conductive polysilicon,
which have typical sheet resistances of 3 and 20 ohms per square,
respectively.
The above analysis does not take into account a benefit of bus 58
connecting the tantalum regions 56 associated with active heating
resistors 24 to tantalum regions 56 associated with inactive heating
resistors 24. As mentioned previously, typically only four adjacent
heating resistors 24 of an array of 200 or more heating resistors 24 are
active at any one time. The voltage swings on the tantalum regions 56
associated with active heating resistors 24 are reduced by a capacitive
voltage divider action provided by the connected, inactive, tantalum
regions 56 and their associated heating resistors 24.
Details of the construction of printhead 10 can be shown with reference to
FIGS. 1, 3 and 4. Heater board 12 includes a silicon substrate 48 with a
major surface 49 on which there is patterned NMOS drive transistors 32 and
logic control section 34. Of course, drive transistors 32 and logic
control section 34 could be fabricate using technology other than NMOS.
Major surface 49, drive transistors 32 and logic control section 34 are
covered by passivation layer 50, which consists of a 1 micrometer thick
layer of silicon dioxide. Glass mesas 52 are formed on passivation layer
50 where heating resistors 24 are to be subsequently placed. Glass mesas
52 consist of 0.9 micrometers thick thermally grown silicon dioxide formed
in the same step in which field oxide regions (not shown) are formed.
Heating resistors 24 consist of a 0.5 micrometer thick layer of
polysilicon that is deposited on passivation layer 50, then patterned and
etched, then patterned and doped with n+impurities in a quantity
sufficient to provide the requisite sheet resistance for an overall
resistance of 200 ohms. Heating resistors 24 are generally positioned
above glass mesas 52, except for their opposite ends 25 and 27 that
contact interconnects 42 and 41, respectively. In this manner, as
discussed in U.S. Pat. No. 4,935,752 to Hawkins, the ends 25 and 27 remain
cooler than the remainder of heating resistors 24, decreasing the failure
of the connections at ends 25 and increasing the transfer of heat from
heating resistors 24 to the ink 57.
Dielectric regions 54 are then formed on top of heating resistors 24.
Dielectric regions 54 can be constructed from silicon dioxide thermally
grown from the polysilicon that forms heating resistors 24, or from
deposited silicon nitride, or from the silicon dioxide followed by the
silicon nitride. Protective regions 56 are formed from a 1 micrometer
thick layer of tantalum deposited on dielectric regions 54 over heating
resistors 24. The tantalum layer is etched off, except over the portion of
heating resistors 24 that reside over glass mesas 52. Dielectric regions
54 are etched off the opposing ends 25 and 27 of heating resistors 24 for
the attachment of interconnects 42 and 41. A first aluminum metal layer is
deposited, patterned and etched to form bus 58 and interconnects 41 and
42. A passivation layer 51 is deposited then etched to uncover portions of
protective regions 56, and to uncover interconnects 41 for the attachment
of power bus 38. Passivation layer 51 consists of a 1 micrometer thick
layer of deposited silicon dioxide. The second aluminum metal layer is
deposited, patterned and etched to form power bus 38 and common bus 46.
For lead passivation, a final passivation layer 61, consisting of 1
micrometer thick silicon dioxide, is deposited, patterned and etched to
uncover terminals 42 and a portion of protective regions 56 to be exposed
to ink 57 in channels 20.
In recapitulation, the present invention relates to an improved thermal ink
jet printhead 10 supplied with a bias voltage and having at least one ink
channel 20, a heating element 24, and an interconnect 42. The ink channel
20 has an open end that serves as a nozzle, and the heating element 24 is
positioned in the channel 20 for ejecting ink droplets 28 from the nozzle
by selective application of current pulses along the interconnect 42 to
the heating element 24. Printhead 10 further includes a conductive
protective region 56 that is positioned adjacent the heating element 24
and that has a portion thereof exposed to the ink channel 20 for
protecting the heating element 24 from ink 57. Protective region 56 is
insulated from heating element 24 by dielectric region 54. Printhead 10
also includes means for connecting the bias voltage to protective region
56, such as bus 58, and means for providing a return path for the bias
voltage, such as conductive ink 57 and grounded sensor contact 27
contacting ink 57. Preferably, protective region 56 includes a layer of
tantalum, and bus 58 is made of aluminum.
Many modifications and variations are apparent from the foregoing
description of the invention and all such modifications and variations are
intended to be within the scope of the present invention.
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